The value of x is zero. In floating-point formats like IEEE 754, where zero can be signed, this value is also returned if
x is negative zero.

FP_SUBNORMAL

Numbers whose absolute value is too small to be represented in the
normal format are represented in an alternate, denormalized format
(see Floating Point Concepts). This format is less precise but can
represent values closer to zero. fpclassify returns this value
for values of x in this alternate format.

FP_NORMAL

This value is returned for all other values of x. It indicates
that there is nothing special about the number.

fpclassify is most useful if more than one property of a number
must be tested. There are more specific macros which only test one
property at a time. Generally these macros execute faster than
fpclassify, since there is special hardware support for them.
You should therefore use the specific macros whenever possible.

This macro returns a nonzero value if x is a signaling NaN
(sNaN). It is based on draft TS 18661 and currently enabled as a GNU
extension.

Another set of floating-point classification functions was provided by
BSD. The GNU C Library also supports these functions; however, we
recommend that you use the ISO C99 macros in new code. Those are standard
and will be available more widely. Also, since they are macros, you do
not have to worry about the type of their argument.

This function returns a nonzero value if x is a “not a number”
value, and zero otherwise.

NB: The isnan macro defined by ISO C99 overrides
the BSD function. This is normally not a problem, because the two
routines behave identically. However, if you really need to get the BSD
function for some reason, you can write